XPS was employed to study analytically the material chemical composition, the nitrogen content and the its bonding environment.
The survey spectrum recorded on the nanotubes sample shown in figure REF  has three main lines: the C1s line of carbon around 285 eV as the most prominent line, the N1s line of nitrogen around 400 eV and oxygen around 530 eV. The last one is related to the minimal oxygen remaining of the surface originated during the purification procedure and also the air manipulation of the sample before the measurements. This oxygen may be reduced with annealing at high temperature, but we refrained from doing this treatment to avoid causing damages to the doping species. Important to notice is the absence of any metals, metal oxides, filter residues or other contaminations in the survey, indicating the high quality of the N-SWCNTs film. Note that the only signal that appears on the survey that could represent a foreign element corresponds to gold which is related to the sample holder of our spectrometer. Apart from that, no other signals have been detected within the experimental limit. The line shape analysis of the C1s line is shown in the REF  on the top figure, where the main peak at 284,5 eV is related to the carbon with sp2 configuration. The broad peak at 285.6 eV is related to the C-N bonds, which is the first evidence that nitrogen is actually incorporated in the carbon lattice [1]}, [2]}. One of the main goals of this work is to unambiguously confirm the effective presence of nitrogen in the nanotubes which has been successfully proved by the N1s core level measurements, taking into account the different atomic cross sections of C and N. Considering all types of possibilities in which N bonds, we observed a total amount of 2.56% N. This value has been estimated considering the ratio of the areas of the peaks C and C-N in the C1s line. Furthermore have analyzed the different types of N bonding environments along the wall of the wall of the SWCNTs, which has been done taking into account previously reported work [3]}, [4]}. The N1s line is shown in the lower part of REF , and it is mainly composed of four types of nitrogen-functional groups: pyridinic (\(\simeq \) 398 eV), pyrrolic (\(\simeq \) 399 eV), substitutional (\(\simeq \) 400,7 eV ) and nitrogenated gaseous compounds or N2 gas molecules (\(\ge \)  402 eV) which are present at the nanotubes surface. Performing the line shape analysis, a high value of 1.55% substitutional sp2 nitrogen and of 0.4% of pyridinic nitrogen was observed. This value is above most of the other reported values for N-SWCNTs. Note that the XPS survey spectrum show the absence, within the experimental limit of 1%, of any magnesium line, that could derive from the catalyst used. This is a confirmation of the favorable outcome of the purification process.
